Sharks: at the nexus of biology and mechanics
Without sharks, the world’s marine ecosystems would lose their balance. Populations would rapidly fluctuate and the entire marine food chain would be reshuffled. This is why CELS Professor Cheryl Wilga is so dedicated to better understanding and protecting these creatures. According to Wilga, these predators are the scavengers of the ocean. They often eat the sick and dying, and thereby keep the fish stocks healthy. They play an essential role in our oceans.
Primarily a laboratory-based scientist, Wilga keeps large sharks, often three to five feet long, in her tanks on the University of Rhode Island’s Bay Campus. There, with URI’s open seawater system, her sharks enjoy continuously refreshed saltwater pumped directly from the Narragansett Bay. This system, in addition to other state-of-the-art technologies, allows Wilga to study the swimming and feeding mechanisms of multiple shark species.
The experiments Wilga conducts with cutting-edge technologies are pushing the boundaries of present-day shark knowledge. In particular, electromyography, sonomicrometry, particle image velocimetry, and high speed video are used to match nearly invisible muscle, skeletal, and water movements with observed shark behaviors.
“You really can’t see much anatomy from the outside of a shark because they are so compact and hydrodynamic,” Wilga said.
Yet, lab techniques enable researchers to look past the skin layer. Often beginning with electromyography, Wilga is able to measure the electrical activity of different shark muscles. Tiny wires, about the width of a human hair, are implanted in the shark and record when a muscle is actively contracted.
Similarly, when focused on the skeleton, Wilga utilizes two-millimeter long, sonomicrometry crystals, to monitor the distance between skeletal points. Each crystal is attached at a specific location where it continuously receives and transmits sound signals. By recording the time between each signal and knowing the speed of sound in water, Wilga can convert the time measurements to distances. This, in combination with electromyography and high speed video, provides a clear picture of how the shark moves.
However, to understand the effects of water on these motions, Wilga must employ one more technique: digital particle image velocimetry, or DPIV. This method relies on an advanced laser and computer system that tracks particles suspended in the water. By recording the paths of these particles, data about the water’s velocity can be turned into video (see the example of a suction feeding shark).
Additionally, under a new grant, Wilga is diving even deeper into the material structure of shark skeletons.
“Today we know that sharks have skeletons made out of cartilage, like in our ears and nose,” Wilga said.
Still, some sharks can bite sea turtles in half. How is this possible, you may ask? Well, according to Wilga, the outside of the jaws and supporting structures of a shark are mineralized. This means that tiny squares of harder material form on the outside of the cartilage, making it stronger but still flexible.
“So it’s not bone, but mineralized blocks connected by ligaments,” Wilga said. “It looks like tiny kitchen tiles.”
Currently, very little is known about the properties of this mineralized cartilage. Therefore, Wilga is testing the strength and flexibility of the material in hopes of discovering why it was favored throughout sharks’ 450 million year evolutionary history.
“They’ve been around much longer than any other vertebrate that is living today,” said Wilga. “And there is a huge diversity of forms, feeding behaviors, ecologies, and habitats. That is why I am studying how and what they came from. It’s fascinating to see how they evolved so many different ways of living in modern times.”